1. Field of the Invention
The present invention relates to an air flow rate measuring device that is disposed in an intake passage to an engine to measure a flow rate of air suctioned into the engine.
2. Description of Related Art
Conventionally, a thermal-type air flow rate measuring device that is disposed in an intake passage to an engine to take in a part of air flowing through the intake passage, and that measures a flow rate of air suctioned into the engine by generating a heat transfer phenomenon between the device and the air which has been taken in, is widely known. This previously proposed air flow rate measuring device includes a housing that defines an internal flow passage, through which the taken-in air passes, and a sensor chip that is disposed in the internal flow passage. The measuring device generates a heat transfer phenomenon between the device and the taken-in air by the sensor chip, and produces an electrical signal according to an air flow rate.
In recent years, in the production of the air flow rate measuring device, for convenience in production, a method, whereby a sensor assembly having a sensor chip is assembled; the sensor assembly is inserted in and fixed to a housing; and the sensor chip is exposed to an internal flow passage, is proposed (see, e.g., JP-A-2010-008225: in JP-A-2010-008225, the sensor assembly is described as a “thermal flow sensor”, and the housing is not particularly described).
The sensor assembly is a component having the sensor chip, and includes a circuit chip which processes an electrical signal that the sensor chip generates, and an external terminal for outputting the electrical signal processed by the circuit chip to, for example, an external electronic control unit (ECU).
The circuit chip and the external terminal are incorporated into the sensor assembly by insert molding, for example, and the sensor chip is fixed to a holding portion formed at the time of insert molding by an adhesive to be incorporated into the sensor assembly. The sensor chip and the circuit chip include various kinds of elements made of a semiconductor film. Generation of a heat transfer phenomenon with air, and generation and processing of the electrical signal are carried out by energization of these elements.
Fixation of the sensor assembly to the housing is by a method whereby a linear expansion of the housing easily transmits to the sensor assembly, such as bonding by the adhesive. Accordingly, due to a linear expansion difference between the sensor assembly and the housing, stress may be applied to the sensor chip or the circuit chip. As a result, if a strain is generated in the element, a resistance value of the element varies due to a piezo resistive effect, and the electrical signal obtained by the air flow rate measuring device thereby has an large error.
For this reason, a configuration that can block the transmission of linear expansion between the sensor assembly and the housing to reduce the stress applied to the sensor chip or the circuit chip is required for the air flow rate measuring device produced by inserting and fixing the sensor assembly in the housing.